I made a thread about this already but want to try and explain it better.

So my truck is a 91 F250 with the 351w and zf5. The issue I'm having is the motor will not rev past 3k rpm in any gear or neutral. It also seems to be lacking power especially up hills. If I hold the gas pedal the engine will get to about 2800 rpm and wont go any further, if I continue to keep it pressed it will fall to about 1500 rpm and if you watch the tach the needle will just bounce off 1500, almost like a rev limiter.

heres a video of the issue:
https://streamable.com/3g8sd

I'm on the gas throughout the whole video, the point where I let off is when its bouncing off 1500 and jumps back up to 2800.

To me it sounds like the cat but I tested it using a vacuum gauge and it passed with flying colors, is there still a chance its the cat?

I also pulled codes and got a little confused, 111 came up multiple times from what I remember? But I'm pretty sure 334 came up later. Something about the EGR valve pos. sensor? Would that cause an issue like this?

Other than that all I've done is replaced air and fuel filters and spark plugs.

If you have any ideas please let me know.

 

Clogged cat or weak fuel pump, check fuel pressure under load before you do anything else.

 

Vacuum gauge will show any exhaust restriction right away, cat or not, so that's not it.

 

Just to be sure we have really ruled out exhaust restriction, HOW did you test is using a vac gauge?

 

If its not the cat then the timing is retarded or no fuel. check fuel pressure at the rail with and without vacuum.

pull the spout and check the timing. put it back in and make sure timing is advancing.

 

I checked vacuum by attaching the gauge to a port on the intake, blipped the throttle and it fell to almost 0 and when I let off it jumped right back to 20 something and held steady there. Then I reved to about 2k and held it for awhile fully expecting it to start dropping but it didn’t move at all from the green.

I’ll have to rent a fuel pressure gauge. I have a question about fuel pumps tho. Doesnt my truck have 2 pumps since it has 2 tanks? The issue occurs when I run it on either tank and the issue occurred suddenly as far as I know. Definitely wasnt doing it when I bought the truck, as I had it over 3k rpm.

Wouldn’t the truck run far worse if it was a timing issue? Truck fires right up and idles fine, accelerates smooth just slow.

 

Another thing is I always hear the pump prime when I turn the key and I even hear it running if I’m outside the truck while the engine is running. Maybe that doesn’t mean everything but a pretty good sign for the pump(s)?

 

It just means the pump is running when it should.

Vaccum check for exhaust restriction - compare your vacuum reading at idle to that at 3000 RPM. The 3000 RPM reading should be as much or more than the idle reading. You are correct that you hold it at the higher RPM for a little while to make sure the vacuum is not dropping.

 

Report back with fuel pressure, but since it happens on both tanks (same ram limit on both tanks) you likely have a clogged cat or a clogged fuel filter. Chances of both pumps being weak to the exact same degree is almost impossible. There are other possible culprits, but let's start with what it is most likely.

 

Would 2k vs 3k rpm make much difference in the vacuum drop test? The needle didn't drop at all at 2k. Really was hoping it was the cat but it seems to be fine. I already replaced the fuel filter, no change. and yes truck runs exactly the same on both tanks so pumps unlikely? I guess I'll just check fuel pressure but won't be able to till this weekend probably.

 

Heck I don't know, 3000 is what I was taught back in HS auto mechanics class in the 70s. But we are talking about a 50% increase, so I would think 3000 would make the problem present itself sooner.

 

so holding the pedal all the way down produces this. if so then you really need to pull the cat and go for a drive.

 

The vacuum during a Rev test won't change substantially because it doesn't take much opening of the throttle blades to achieve increased rpm, because there is no load. The vacuum while driving changes substantially as load is increased, because the throttle blades will be open much more to maintain or achieve increased rpm. There *would*, however, be a substantial drop in vacuum during the rest Rev test if there were an exhaust restriction that was limiting rpm. You should also be able to read very little vacuum if you can hold the throttle blades at wot at the "rpm limit" of 3000. To test for a clogged cat via a Rev vacuum test, you'll have to test with partial throttle where vacuum should still be high, and recognize the loss of vacuum.

Fuel pressure fuel pressure fuel pressure. It could be as easy as a fuel filter, but I can't rule out a clogged cat from the info above alone.

 

Doesn’t matter how far I press the pedal, it does it at the exact same rpm no matter the circumstance.

I replaced the fuel filter, it passed the vacuum test, should I try it while driving? And both pumps sound and operate the same. I’m just going to get a fuel pressure reading but what else can it really be?

 

Have you checked to see if the pedal makes the throttle plates open up fully?